Question: $ A = \left[\begin{array}{rr}4 & 0 \\ 0 & 1 \\ -2 & 0\end{array}\right]$ $ E = \left[\begin{array}{rr}2 & 2 \\ 2 & 0 \\ -2 & -1\end{array}\right]$ Is $ A+ E$ defined?
Solution: In order for addition of two matrices to be defined, the matrices must have the same dimensions. If $ A$ is of dimension $( m \times  n)$ and $ E$ is of dimension $( p \times  q)$ , then for their sum to be defined: 1. $ m$ (number of rows in $ A$ ) must equal $ p$ (number of rows in $ E$ ) and 2. $ n$ (number of columns in $ A$ ) must equal $ q$ (number of columns in $ E$ Do $ A$ and $ E$ have the same number of rows? Yes Yes No Yes Do $ A$ and $ E$ have the same number of columns? Yes Yes No Yes Since $ A$ has the same dimensions $(3\times2)$ as $ E$ $(3\times2)$, $ A+ E$ is defined.